May 29, 2007

CSP as Energy Storage

Another way to look at CSP, as a Stored Energy System that complements PV, rather than competing with it.

Electricity is a bit of a strange beast, kind of like the Internet, in that generating power does not need to be concentrated in one place. For example, if a big PV plant only outputs while the sun shines, some other plant, perhaps wind, gas, or stored energy hundreds of kilometers away can take up the slack when it gets dark. Since thermal storage is the main intrinsic advantage of CSP compared to PV, this geographic/power source decoupling makes many other non-obvious possibilities practical.

We all know that solar and wind have a big problem: you can't rely on them. The key to this is energy storage - store it up for when the sun goes down or the wind dies. There are lots of storage schemes, the main problem being that it is difficult and expensive to store electricity. Heat is easier.

The heat storage of CSP may be an economical storage system, in fact, why not just use them as storage systems, only generating electricity when the lower cost PV and wind systems are dark? This would entail making the molten salt storage much larger than currently planned (usually about 8 hours storage). The molten salt retains its heat for a long time, I recall seeing something like a 1 degree per day loss in a hot tank. If you make the storage big enough, it could run the turbines for days, even in the rain, making up for some of those dark PV panels and stopped wind turbines. Acting as a renewable storage scheme may make CSP much more valuable, rather than simply competing with PV.

As to how much energy can be stored, I don't think there are any practical limits, just economic ones. The 8 hour storage was chosen because this is about how long the Spanish stay awake after the sun goes down. The molten salt (special low melting point salt) is pumped from a cold (~250C) insulated tank, heated using the oil in the solar troughs and pumped into the hot (~500C) tank. When you need electricity, you reverse the flow and heat up water for the turbine. A dedicated "peaking" CSP with massive thermal storage like this would only generate electricity at high $ value times, and storing heat at other times for when it is needed.I think this may be practical, but who knows? It would compete with other storage schemes like pumped hydro, CAES and flow batteries. Someone would have to run the numbers. Are you listening Flagsol/Solar Millenium?

I do know, that these storage systems are massive. The Grenada plant is two 50MW plants side by side (to comply with the 50MW maximum for the Spanish Feed in law). For 8 hours of storage, this is 2X50X8= 800MWHours - that's a big battery! Of course it cost a lot too, something like 150Million euros each.

Also, I wonder if the resulting system would be simpler than the current one that runs the boiler and the storage in parallel. If all you want to do is run the boiler off the stored heat, you could just have one heat exchanger for the oil to salt transfer, and one other one for the salt to water transfer. Currently the Flagsol system in Spain uses a more complicated system that heats both the water and the salt in parallel so the steam turbine can run off solar and/or stored heat and mixes and matches thermal flows to the solar output and needed thermal storage.

I envision a solar complex that uses cheap PV for generating electricity during the day, coupled with a CSP Energy Storage (CSPES?) system for making the complex fully dispatchable, even during night and cloudy days. I know, right now, PV isn't cheap, but someday it will be! This scheme uses both technologies for what they do best - PV for daytime electricity, and CSP for energy storage.

How about wind - would it help for wind? Wind blows somewhat unpredictably, so, yes, any storage system can help even out the spikes and troughs. Use your CSP to absorb energy from the sun and store it. Then when the wind dies, use the stored heat to run the steam generator. The only issue would be that you will still probably throw away the wind energy at night, when no-one is there to use it. Overall though, CSP is probably better as a storage system than as a direct solar system!

5 comments:

Excess wind energy (in the form of electricity) can be treated identically to excess electricity generated from photovoltaics: transform it into heat and store it as heat.

I too would like to see the numbers comparing cost-efficiency of heat-based storage with, in particular, pumped water. Unlike liquid salt, which loses 1 degree per day (say), pumped storage loses no energy at all over time (unless the reservoir leaks ;-).

AWE MOLTEN SALT USED AS A MEANS TO STORE HEAT COULD HAVE MANY ADVANTAGES. IM THINKING OF USING PUMPED AIR IN A WATER TANK TO DRIVE A GENRATOR. THEN TO ENERGIZE THE AIR WITH A MOLTEN SALT HEAT EXPANDER, BEFORE GOING TO THE WIND TURBINES TURNIG THE GENERATOR. TO USE THE EXPANDER EFFECTIVELY IT CAN BE BUILD INTO A GEYSER SYSTEM TO HEAT UP WATER FOR A HOUSEHOLD.